Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ) are widely used as the modulation name when they are implemented for On-Off Keying (OOK). In the case of OOK for optical communications, the signal is carried by a light carrier and the coding is performed as:    (a) Light is emitted during the symbol period when the coded bit is “1”.    (b) No light is emitted during the symbol period when the coded bit is “0”.
However, when consecutive identical bits appear, RZ and NRZ implementations are widely used:    (a) Light is temporary shut (no light emitted) between consecutive coded “1” bits: this is RZ.    (b) Light is not shut between consecutive coded “1” bits: this is NRZ.Apparatuses for implementing the RZ modulation are disclosed in Japanese Unexamined Patent Publications No. H09-230290 and 2004-180343, for example.
In addition, RZ and NRZ can be used in conjunction with phase modulation schemes such as Binary Phase Shift Keying (BPSK), QPSK (Quaternary Phase Shift Keying), combination of Amplitude and Phase Modulation, such as 2ASK-4PSK, Quadrature Amplitude Modulation (QAM), and the like. Consecutively, if RZ is used in conjunction with the said modulation formats, the amplitude is forced to zero between consecutive symbols. In the same way, if NRZ is used with the said modulation formats, the amplitude is not forced to zero between consecutive symbols. Its value is decided according to the constellation map of the said modulation format. Note that from this point no distinction is made between Differential Phase Shift Keying (DPSK) and PSK, as the difference is only in data coding but optical signals are identical. Apparatuses for implementing the RZ-DPSK modulation are disclosed in Japanese Unexamined Patent Publications No. 2003-60580 and 2007-512748, for example.
Depending on the transmission line or condition, one may choose to implement either RZ or NRZ. On one hand, RZ is known has offering better receiver sensitivity, better tolerance to Polarization Mode Dispersion (PMD) impairments, and reduced effects of optical amplitude ripples between symbols. On the other hand, NRZ signals have narrower optical spectra, offering therefore better tolerance to filtering effects, or better tolerance to Chromatic Dispersion (CD).
Pseudo-Return-to-Zero of index n (PRZ(n)) has been introduced as the modulation scheme, where the amplitude of the lightwave signal is forced to zero every n symbols and not forced to zero for other cases. According to Le Taillandier de Gabory et al. (ECOC 2009, paper 3.4.4), PRZ enables to monitor intra-polarization skew for polarization multiplexed signals and also enables to discriminate the multiplex polarizations. Moreover, according to Le Taillandier de Gabory et al. (IEICE Society Conference 2009, paper B-10-85), PRZ enables to enhance to tolerance of optical polarization demultiplexing to first order Polarization Mode Dispersion (PMD).
According to the definition of PRZ(n), PRZ (1) can be identified as RZ and PRZ(∞) can be identified as NRZ. Therefore, in the scope of the present invention, we reduce PRZ(n) to the cases where n is finite and n is greater or equal to 2.
In PRZ(n) format, the amplitude returns to zero between consecutive symbols every n symbols and does not return to zero in the other cases, unless the symbol amplitude is zero or crosses zero. PRZ(n) format is not a RZ format of at a n-time slower clock frequency, as the amplitude is forced to zero and is relaxed at a very steep slope (same slope as RZ). PRZ(n) causes dips on the optical amplitude every n symbols. On the contrary, RZ format at a n time slower clock frequency would cause a n-time slower slope when the amplitude is forced to zero and would cause degradation of the quality of the transmission as the central part of symbols are affected by the slow slope of the amplitude change. Therefore, RZ at an n-time slower clock speed cannot be used as a substitute to PRZ(n).
A proposed way to generate PRZ(n) carving is to use the transitions of a PSK modulator between consecutive trains of n identical symbols. This configuration implies some disadvantages.
First, an additional optical modulator is needed to carve PRZ dips. Moreover, the additional modulator will require also Auto Bias Control (ABC) circuit in order to avoid drift of the bias point, and therefore degradation of the signal quality because of thermal drift or device aging. Therefore, such a scheme will increase both the size and the cost of an optical transmitter implementing PRZ carving.
Second, the additional optical modulator used to carve PRZ dips causes loss to the optical signal. Typically, the insertion loss is in the order of 6 dB. This causes, de facto a loss in optical signal to noise ratio of the emitter, and therefore this affects the quality of the emitted signal. One may want to compensate this loss by using a light source laser with a higher power for the output signal. If such a laser is available, its power consumption will be higher.
Third, the optical modulator used to imprint the data onto the optical carrier and the optical modulator used to cave PRZ dips will be separated by optical fiber. As the refractive index and length of the optical fiber changes with temperature, the synchronicity of both said modulator will be affected by temperature drifts, and therefore the quality of the emitted signal will suffer from temperature changes.
Fourth, integrated modulators for dual polarization (DP) are bound to be a cost effective solution, as encouraged by the standardization activities of the Optical I. Forum (OIF). However, as PSK modulators have a strong polarization dependent loss (PDL), the effective only solution to use a PSK modulator in conjunction with a DP modulator to carve PRZ dips, is to place the PSK modulator before the PRZ modulator. In that configuration, both polarizations will have the same PRZ carving at the same index. In that case, polarization discrimination based on the PRZ dips is no longer possible.
Fifth, in the case of high index n for the PRZ carving, typically when n is equal or greater than 8, carving PRZ (n) dips with a PSK modulator requires a wide and flat bandwidth characteristics for the modulator, as the transmission contains at the same time low frequency components, the long trains of constant amplitude occurring during the n constant symbol trains, and high frequency components, the PSK modulation transmission, by which the dips are curved occurring at the transition between opposite trains of n symbols. However, for 100 G PRZ-DP-QPSK, the baud rate is 25 Gbaud and in such case the PRZ carving may cause degradation on the emitted signal when n is in the order of 8 with standard PSK modulators. Imposing a tighter specification on the flatness or bandwidth of the modulator leads to an increase in cost.
Moreover, one could think of using a high speed Digital to Analog Converter (DAC) in order to control precisely the position of the emitted light signal on the constellation map, when the light signal is modulated by a Cartesian modulator; in order to carve PRZ dips in this manner, one would need to generate a signal coding the modulated symbol and the transition between symbols at the same time. This means that at least two values have to be generated by symbol, requiring a DAC operating for at least twice the baud rate. However, in the case of 100 Gb/s DP-QPSK modulation format, the baud rate is 25 Gbaud, therefore one would require at least a 50 Gb/s DAC to control at the same time the emitted symbol and the transition on the constellation map. These devices are not commercially available at this point, and when they will be, they will be at a high price and consume a high amount of electrical power.
However, there is room for improvement in simplicity, size, cost, emitted signal power, stability and functionality of PRZ carving apparatuses. There is a need for a simple, small-sized, cost effective, low loss, stable and versatile PRZ carving apparatus.